Try this:
Turn off
"Options > Media Network > Add or configure DLNA server... > {select your DLNA server} > Advanced > Bitstream DSD (requires DoP compliant renderer)"
so that MC isn't encapsulating your DSD files in a PCM container, but is sending the original file.
Also, set
"Options > Media Network > Add or configure DLNA server... > {select your DLNA server} > Audio > Mode" to "Original"
so that there is no conversion on MC at all before being transmitted.
Now MC should be just serving the DSD file via DLNA to the Volumio DLNA Client.
If Volumio is capable of receiving and playing that DSD file, how does it go? Any pops at Start/Stop?
If so, then set Volumio to play a little silence at the start and stop of playback, as required. Fixed?
Basically, you are telling MC to encapsulate your DSD file in PCM prior to sending the resulting file to Volumio. Hence, you are possibly observing a transition from PCM to DSD. Stop telling MC to encapsulate your DSD file in PCM, and then Volumio should receive the file, and it will be responsible for all further buffering and processing, along with the Denon.
No, you don't need Bitstreaming from MC to Volumio, as Volumio will be handling all the buffering, timing, and if necessary Bitsreaming to the Denon, if you set that up. But you probably don't need to do that, as the Denon is handling all the decoding/sampling/restoring the file to its original sample rates and timings.
Disclaimer: Of course, I could be completely wrong. I don't have the equipment to check this, and even then I would have to listen for the pops. But I feel there has been a bit of a disconnect in this thread, and perhaps we can get past that. It would be worth reading back through the thread again, as I have, to find the new meaning is some of the posts.